1. Field of the Invention
The present invention relates to a thermally-actuated switch assembly, and in particular to such a switch assembly which may be used to control a damper in an air distribution duct.
2. Description of the Related Art
It is known to provide thermally-actuated dampers in ducts. For example, a damper may be fitted in a duct adjacent to a part of a duct system which penetrates a fire barrier or wall. Such dampers are arranged to close the duct in the event of temperatures within and/or outside the duct exceeding a pre-determined threshold. Dampers of this type are specified where it is necessary to ensure the integrity of the fire wall and thereby prevent the spread of a fire through a duct penetrating the fire wall. With such an arrangement the damper is normally open but closes in the event of an excess temperature being sensed. Other dampers are known which operate in the opposite sense, that is they are normally closed but open when exposed to an excess temperature. Such dampers may be used in situations where there is a requirement for providing a smoke vent which only opens in the event of a fire.
One known damper consists of a steel casing which is interconnected between two sections of a duct. The casing houses a number of interlocking steel blades which can be rotated through 90 degrees between a first position in which the blades are edge-on to the direction of the ducts and a second position in which the blades extend transversely with respect to the direction of the duct. In the first position the blades are spaced apart and air can flow between them. In the second position the blades overlap and form an effective barrier across the duct. Generally the blades are held in the first (open) position to allow air flow through the ducts. The damper blades are held in the open position by means of a spring return actuator. The actuator incorporates a motor which is mechanically coupled to a damper shaft rotation of which controls the position of the damper blades. When power is supplied to the actuator, the motor is energised and turns the damper blades to the open position. The motor as it rotates the blades to the open position also tensions a spring. Once the actuator has been fully reset so that the blades are in the open position, the motor stops and a mechanism holds the spring in its tensioned condition providing the supply of the electrical power to the actuator is maintained. If the supply of electrical power is cut off, the spring is released and the damper shaft is driven to a position in which the damper blades extend transversely relative to the ducts, that is the damper blades are in the closed position.
In the known damper a thermal fuse is incorporated in the electrical supply to the actuator, the thermal fuse being mounted on either the damper casing or one of the ducts connected to the damper casing. Generally the thermal fuse incorporates a single fuse element which is mounted on a probe that extends through an opening cut in the casing or duct. In some arrangements however two thermal fuses are provided, one mounted in use within the duct and one mounted external to the duct. If one of the fuses is exposed to a temperature in excess of a pre-determined limit the fuse assumes an open-circuit condition, thereby cutting off the electrical supply to the damper and causing it to move to a duct closed position.
The electrical supply to the damper may be cut off by other means, for example by switching power off at a control panel to which the fire service might have access, or a simple mains failure. The present invention is concerned however with situations in which a switch assembly incorporating for example a fuse is incorrectly positioned such that the fuse is not exposed to the environment the temperature of which it is intended to sense. This can occur for example because an installer makes a simple error on installing for example a damper system. The error could be as simple as a failure to mount a switch assembly correctly on for example a duct.